1. Field of the Invention
This invention is directed to a power zooming device which changes a lens's focal length by motor. More precisely, it is concerned with a speed control of the zooming device.
2. Description of Related Art
Conventional cameras, such as TV cameras, video cameras, and still cameras come equipped with a power zooming mechanism which changes a focal length by driving a zoom lens with a motor.
In general, however, the relationship between a zoom ring's rotation and a change in a focal length of such zoom lenses is not proportional. The curve shown in FIG. 7 shows a representative example of this relationship between the focal length and its rotation. In such a zoom lens, even if zooming is done at a constant rotation, the focal length changes slowly at the wide-side and quickly at the telescopic-side.
From the user's point of view, zoom lenses with a different rate of change in the focal length is not favorable in terms of maneuverability. If a constant change in the focal length, as shown in the straight line in FIG. 7, can be achieved, maneuverability is much better.
However, the curving trend of the change in the focal length of a zoom lens is caused by the zoom ring cam design to achieve its constant rotational torque. When the design is such that the straight line, as shown in FIG. 7, can be achieved, the zoom ring's rotational torque changes, causing an unfavorable situation, too.